1. Field of the Invention
The present invention relates, in general, to fluid heating apparatus and methods, and more particularly, relates to thin film heaters formed by the growth or deposition of an electrically conductive thin film on a supporting electrically insulating substrate and the use of such thin film heaters to heat fluids.
2. Description of Related Art
There are several ways of heating fluids, but most employ a heater element operating at a relatively high temperature because the contact time between the gas or liquid and the heating element is often minimal. Thus, in fluid heating there is often a trade off between fluid contact time and heating element temperature in which temperature is often easier to increase as a practical matter than is contact time. This trade off is present for a wide variety of heaters including thin film resistance heaters.
Resistance heating of fluids using thin films deposited upon mechanically supportive substrates is well known in the art. Typical of such prior art devices are the thin film heating devices shown in U.S. Pat. No. 5,616,266 to Cooper (the “Cooper '266 Patent”). The resistance heaters disclosed in the Cooper '266 Patent includes a flow-through louvered heater in which a gas, usually air, flows through louvers having a thin film heater deposited thereon.
In the louvered heater, of the Cooper '266 Patent, however, most of the fluid passing through the heater is not actually in direct contact with the thin film. The boundary layer of gases contacts the film on the louvers, but most of the gases simply flow through the openings defined by the louvers.
Typically, thin film heaters, and specifically the heater of the Cooper '266 Patent, are deposited on smooth substrates such as glass, quartz alumina, mica, ceramic, porcelainized steel materials, etc. These types of materials are well suited as they provide an electrically insulating surface for the electrically conductive thin film heater to be applied to, as well as possessing desirable high temperature self-supporting physical properties necessary for a heater. A common feature of these prior art substrate materials is that they are non-porous and have a smooth surface finish. Smooth substrate surfaces minimize turbulence within the film deposition reactor, which if present, can lead to uneven thickness of the deposited film, and/or other undesirable qualities.